1. Field of the Invention
The present invention relates to acoustic-propagation-time measuring apparatuses capable of performing high-accuracy analytic measurement of an acoustic propagation time in a fluid or solid. In particular, the present invention relates to an acoustic-propagation-time measuring apparatus capable of measuring the distance to an object to be measured and the distance distribution by measuring an acoustic propagation time.
2. Related Art
One conventional example of an acoustic-propagation-time measuring apparatus is a sonar device described in Japanese Patent Laid-open (KOKAI) Publication No. 2003-139855.
The sonar device described in this reference includes transmitting and receiving means having a plurality of ultrasound transducers. The sonar device emits an ultrasound signal from this transmitting and receiving means to an object and receives a reflected wave with the ultrasound transducers, realized by a plurality of receiving elements, to detect the direction and the position of the object from a phase difference and an ultrasound propagation time of the ultrasound receiving signal for display by display means.
This sonar device, used as, for example, a fish finder, is constructed so as to be capable of detecting the two-dimensional position of a reflecting object such as a school of fish. This sonar device can measure the direction and position of an object to be measured, such as a school of fish, by measuring a phase difference and an ultrasound propagation time of a signal.
Referring to FIG. 10, if a target object 1 to be measured has a bump or irregular (having protrusions and recesses) measuring surface thereon, is complicated in shape, or suffers from a complicated defect, the bump surface or shape cannot be measured correctly or the defect on the target object 1 cannot be accurately inspected with a known sonar device.
When an ultrasound is emitted from an ultrasound transducer 2 to the target object 1 disposed in a fluid and having thereon a bump or irregular portion (i.e., protruded stage or portion), two echo signals are returned, including an echo signal e1 reflected from an upper surface 1a of the bump and an echo signal e2 reflected from a lower surface 1b of the bump, as shown in FIG. 11A. If both echo signals e1 and e2 are separated from each other as shown in FIG. 11A, an acoustic propagation time T1 and a time difference t1 between the echo signals e1 and e2 can be measured correctly. Thus, the distance to the target object 1, the distance difference (gap or step), and the bump distribution can be measured with high accuracy by multiplying the measured acoustic propagation time T1 and the time difference t1 by the acoustic velocity.
Referring to FIG. 11B, however, if the target object 1 to be measured has only a small bump or if both the echo signals e1 and e2 overlap by being only slightly shifted, i.e., the ultrasound waveform components overlap each other, an acoustic propagation time difference t2 cannot be extracted correctly for accurate measurement, and consequently, the distance, distance difference (gap), bump distribution, or shape in relation to the target object 1 cannot be measured with high accuracy.